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Red blood cells. Light micrograph of a group of red blood cells (erythrocytes). During its short life (about four months) a red blood cell covers about 15km every day for a total of 1500km. They transport oxygen from the lungs to the tissues where it is necessary for all metabolic processes. The carbon dioxide produced in these processes is returned by the red blood cells to the lungs where it is exhaled. In a litre of blood there are normally 5000 billion red blood cells. A mature erythrocyte has no nucleus and the shape of a biconcave disc with a diameter of about 7 microns.